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Comprehensive Description

General Description

The shell is gigantic for a mussel, measuring up to 360 mm long, and is rather thin and fragile. The shell is quite variable in outline and tumidity, but is equivalve. Fully grown specimens are markedly curved and are occasionally slightly twisted, with most shells being slightly inflated behind the middle. Juvenile specimens have shells that are already very slender, and have a nearly straight ventral margin when less than 110-130 mm. The beaks are subterminal, occurring at about one-fifth of the total shell length in adult specimens. The anterior part is rather narrow and tumid, becoming somewhat protruding and nose-like in fully-grown specimens. The anterior margin is narrow but evenly rounded. The ventral margin is nearly straight to concave in juveniles and half-grown adults, and becomes markedly concave in the anterior half in adults, occasionally with a crease-like concave angle below the umbos. The postero-ventral margin is broadly rounded, whilst the postero-dorsal margin is well convex. The postero-dorsal corner is rounded to indistinct. The ligament plate is almost straight to slightly convex. The exterior surface has more or less developed, irregular growth lines. There are occasionally very faint broad radial undulations that are reminiscent of the fine striation of the mytilid genus Brachydontes visible on the broad posteriormost part which are slightly reflected on the inside of the shell. The umbo is very broad and is more prominent in fully grown specimens than in juveniles.

The periostracum is strong and smooth, with a dark chestnut brown colour. It is glossy to somewhat dull, and is without any periostracal hairs. However, there may be some byssal endplates of other specimens scattered over the valve.

The hinge is without a tooth. The ligament is very strong, and extends over about five-sixths of the postero-dorsal corner and ends in a posterior taper. There is a subligamental shell ridge that is more or less faint from under the umbos to one-third or one-half of the ligament. The anterior adductor scar is oval, and is situated in front of the umbo. The posterior adductor scar is rather large and is somewhat rounded. It is united with the posterior scar of the posterior pedal and byssus retractor muscle. The anterior scar of the same muscle is separated and is situated under the final third of the ligament. The anterior byssus retractor muscle scar is under the beak, in the middle part of the umbonal cavity, and is visible only from posteriorly and ventrally, but not at a lateral view of the interior. The pallial line is slightly S-shaped, and is about parallel to the ventral margin.

The shell without a periostracum is a dull-whitish colour, and the interior of the shell is nacreous-white.

The larval shell (protoconch II, observed on a single specimen of 11 mm length) is about 350 um long and high, coloured reddish brown, sharply separated from the whitish postlarval shell.

The ctenidia are very large, occupying more than three-quarters to nearly four-fifths of the shell length. The outer demibranch is slightly shorter anteriorly than the inner demibranch. The ascending lamellae of the outer demibranch are anteriorly fused for a very short distance to the mantle, whilst those of the inner demibranch are fused to the visceral mass. There are no muscular longitudinal ridges on the mantle and visceral mass where the dorsal edges of the ascending lamellae attach. Connection bars between the free edges and gill axes are absent. The filaments are broad and fleshy.

The inner mantle folds separate along the whole ventral margin length from the anterior adductor to the posterior margin, with their edges frilled along the posterior fourth and becoming smoother towards the anterior. On the anterior end the inner mantle folds pass from ventrally over the anterior adductor muscle up – and forward along the anterior margin for a short distance, then folding down – and backward to pass again the lower end of the anterior adductor muscle towards the ventral margin. The valvular siphonal membrane is very short, narrow and quite thin. It is situated just anterior to the siphonal opening and bears a well-developed papilla.

The foot is small in fully grown specimens, and is rather slender, with a length of 50 mm in a 340 mm shell (byssus orifice included). The foot-byssus retractor muscle complex is prolonged like the shell with a comparatively short anterior retractor. The posterior byssus retractors consist of two diverging muscle bundles that have a common base at the base of the byssus. The anterior bundle is short and very broad, and arises to the attachment point on the shell interior at an angle of about 40-45°. The posterior bundle is very long and thinner than the anterior bundle, and passes at a very low angle (~15°) to the longitudinal shell axis towards the attachment point above and in front of the posterior adductor. The posterior foot retractor is very thin, and arises from the anterior side of the foot base well in front of the base of the byssus retractor muscles, passing the outer side of the anterior retractor towards the anterior bundle of the posterior byssus retractor, reaching the shell inside closely appressed to it to a short part of its length. The labial palps are irregular and broadly triangular in shape. In adult specimens they are very small, with the anterior two about 5 mm long, and those of the posterior pair about 11 mm. They are more developed in juvenile and half-grown specimens.

The stomach is extremely small for the animal’s size, and is very elongate, with thin walls and a small and hardly recognisable anterior chamber. The posterior chamber is longer and broader. In the examined specimen only four entrances of the digestive diverticula were visible; two at the end of the anterior chamber, and two in the posterior chamber. The style sac and midgut are conjoined. The major typhlosole passes from the midgut along the floor of the posterior chamber to just behind the anterior chamber. On the posterior left side there is a very shallow depression that corresponds to the left pouch. A gastric shield was not seen. The midgut runs straight and median from the stomach towards the posterior to under the ventricle, and then enters it without any coiling or loop.

The heart is situated well to the posterior, with large auricles that are fused posteriorly and which stretch out forward in lobes in front and around the anterior bundle of the posterior byssus retractor.